A common way of providing precision controlled weapons is to point at a target by means of a laser beam and allow a flyable weapon such as a missile, bomb or the like to guide itself towards the laser spot. Consequently a laser target seeker is required, being able to distinguish the reflected laser beam in relation to the background. By laser target seeker a target seeker is intended, the task which is to detect reflected laser beams from a target pointed out in the terrain by means of a laser designator and to determine the received radiation on the respective detector element in order to determine the origin of the laser beam. The laser target seeker is also sometimes referred to as a semi-active laser sensor or a SAL-sensor.
The task of the target seeker is to inform where in the field of view of the target seeker the reflected laser spot is present. A common way of achieving this is to use a quadrant detector. A quadrant detector consists of four separate detector elements placed edge to edge in one plane, each of the detector elements being able to view a quadrant of a certain visual field. By measuring and comparing the signal intensity from the four quadrants, i.e. the amount of optical power ending up on each of the four elements, it is possible to determine where the point of balance of the laser reflex is located and thus in which direction to guide e.g. the missile. When the intensity of the signal is equally strong from all four elements the laser reflex is in the centre of the detector and the missile will hit in the middle of the laser reflex. A problem with the quadrant detector is that the accuracy is quite low, which might lead to the missile or the like missing the target.
If a higher accuracy is required in designating the target, the reflex may be focused to an area smaller than the quadrant detector. Then the centre point may be found with higher accuracy but when the reflex is located outside the centre it is not possible to determine how far from the centre it is. This leads to a guide operation which sometimes is referred to as “bang-bang”, i.e. strikes are made between the end positions until the reflected beam is in the centre region, at which point a better control may be provided. This means however that a pour aim of the target is the case until the reflex is in the centre. Before that only the quadrant in which the target is located is known.
A more advanced alternative is to use a two dimensional matrix of detector elements to view the entire field of view, e.g. a detector matrix having 4×4 elements. The laser reflex will hit one or more of the elements. By comparing the intensities of the signals the position where the reflex is located may be calculated. An advantage is that a more precise determination of the position of the target in the field of view is achieved. It is also possible to create multiple “images” taken one after the other and use image processing routines to follow and chose among target candidates.
There are however some limitations in using a two dimensional matrix having more than four elements. In a quadrant detector a wire is drawn from the periferal edge of each element to a corresponding amplifier, and the wires consequently do not affect the incoming laser beam hitting the detector elements. Each amplifier can further be located close to the corresponding wire. In the case of a detector matrix having more elements, e.g. 4×4 elements, however, the wires drawn from inner elements will affect the area of the elements, i.e. the fill factor. They are usually drawn along a path where the edges of two adjacent element meet, affecting the fill factor and consequently the accuracy. The amplifiers connected to the wires drawn from the inner elements need to be placed further away, leading to a higher capacitance, which increases the noise and consequently affects the accuracy.